At a manufacturing step of a semiconductor device, etching processing is an important step for pattern formation, in which processing of selectively etching and removing a silicon nitride film out of the silicon nitride film and a silicon dioxide film formed on a substrate while leaving the silicon dioxide film is required. As a method of performing such selective etching processing of the silicon nitride film, a process of using a phosphoric acid aqueous solution of a high temperature (140° C. to 180° C.) as an etchant is known.
In the process of using a high-temperature phosphoric acid aqueous solution, a plurality of substrates having silicon nitride films and silicon dioxide films formed thereon, respectively, are dipped into a treatment tank in which a high-temperature phosphoric acid aqueous solution is retained to perform the selective etching processing of the silicon nitride films, for example. At that time, because of characteristics of the phosphoric acid aqueous solution, a slight amount of the silicon dioxide films is also etched. To address this problem, for example, a silicon compound is added to the phosphorous acid aqueous solution to enhance the silica concentration therein so as to increase the etching selectivity of the silicon nitride films against the silicon dioxide films. An increase in the etching selectivity of the silicon nitride films can suppress etching of the silicon dioxide films.
Generally, when the etching processing of the silicon nitride films is performed using the phosphoric acid aqueous solution, silica is accumulated in the phosphoric acid. If the silica concentration in the etchant is too low, the etching rate of the silicon dioxide films becomes high and the etching selectivity of the silicon nitride films becomes low. If the silica concentration is conversely too high, a problem that silica adheres to the treatment tank or clogs a filter occurs. Accordingly, when the etching processing using the phosphoric acid aqueous solution is to be performed, various silicon-based additives are used to adjust the silica concentration in the phosphoric acid to be fallen within an appropriate range corresponding to a treatment object.
In recent years, in a manufacturing process of a three-dimensional memory device, a stack film is formed on a wafer by alternately staking silicon dioxide films and silicon nitride films. To form word lines, select gates, and the like, the silicon nitride films in the stack film are selectively removed through a treatment with phosphoric acid having a predetermined silica concentration adjusted in advance.
However, in the three-dimensional memory device, because the amount of etching of the silicon nitride films is large, silica reaches the saturation concentration during the phosphoric acid treatment and deposits between layers of the silicon dioxide films, which leads to difficulty in forming wiring layers between the layers of the silicon dioxide films. With staking of more layers in a future three-dimensional memory, a larger etching amount of the silicon nitride films and a narrower margin for deposition of silica due to thinner silicon nitride films are expected and there is a concern that the problem of silica deposition becomes more serious. Therefore, to manufacture a three-dimensional memory with a high density at a high yield ratio, suppression of the silica deposition is demanded.